1. Field of the Invention
The invention is directed to an RF transmission/reception antenna for an open MR system, of the type wherein the bottom and top poles of the basic field magnet are connected to one another by two columns.
2. Description of the Prior Art
Conventionally, antennas in stripline form have been employed as RF transmission/reception antenna. The antenna is composed of striplines that are shorted at the end with one or more resonant capacitors relative to a ground or shield plane and thus are balanced to resonance. A significant disadvantage of this antenna structure is the poor efficiency due to the slight distance between the actual conductors and the shield surface. Increasing the distances is in fact conceivable but the space for the patient is then also reduced as a result.
An object of the present invention is to provide an RF transmission/reception antenna for an open RF system that, with a simple structure and while maintaining the largest possible acceptance space for the patient, has a higher efficiency than known stripline antennas.
This object is inventively achieved in an RF antenna having an electrically conductive connecting loop that surrounds the patient space and that is formed by the basic field magnetic system with the connecting columns, and having a shortening capacitor inserted in the loop.
The basis of the present invention is that the basic field magnet itself is integrated into the antenna and its metallic surface is utilized as an active conductor. The insertion of shortening capacitors is required for utilization of this conductor loop and MR antenna. The interruption of this loop for the direct insertion of such resonant capacitors is a solution that, however, is frequently not expedient in practice. For example, the electrically conductive connecting loop can be separated at a location, particularly in the region of a column, for the insertion of the shortening capacitor.
A solution has proven more expedient to attach a resonant cavity, having a capacitor, to at least one of the connecting columns, so that a capacitance is transformed into the loop, the tuning to resonance and an adaptation to the MR transmitter is possible as a result. The efficiency of the inventive resonator antenna is significantly higher than that of the conventional solution with striplines. Another advantage is the elimination of the vertical space that was required for the conventional body resonator.
The feed can ensue directly at the resonant cavities at the columns in the immediate proximity of the space provided for the matching units. Given a tilted magnet (such as, for example, the type referred to as a xe2x80x9cdouble donutxe2x80x9d), this inventive principle of the structure of the RF antenna having a loop surrounding the patient acceptance space can, of course, be likewise applied. The structure can be designed such that the upper brace contains a capacitive impedance that can be realized by a blocking or rejection circuit. In this case, a division of the vacuum vessel would also be possible at the top, so that a doubled top would no longer be required at all, which would then yield a significant gain in bandwidth and efficiency.
A slight disadvantage of the inventive structure of the RF antenna is the large excitation volume of the antenna, which is prescribed by the shape of the magnet housing. In order to reduce ambiguity artifacts in the MR images, in a further embodiment of the invention auxiliary resonators generating an opposing field are laterally arranged at the gradient coil for the spatial limitation of the RF field in the region of the patient axis. These auxiliary resonators, which can be constructed as striplines shorted at both ends and that can be excited with the main resonator due to the inductive coupling, generate a B-field that is directed opposite to the main field and compensates the field in the outside space. As a result, a rapid field drop occurs in the patient direction and the ambiguity artifact is reduced.